1. Field of the Invention
The present invention generally relates to a blood component monitoring system for measuring and/or intermittently monitoring the concentration or partial pressure of chemical components contained in blood and, more particularly, to a system for measuring and/or monitoring the concentration or partial pressure of chemical components of interest contained in blood by the use of a flow-through cell, into which blood is intermittently extracted through an indwelling catheter or cannula inserted into the blood vessel, and temperature and chemical component sensors both diposed within the flow-through cell, with no need to drain the extracted blood outside the system.
2. Description of the Prior Art
The continuous or intermittent measurement of chemical properties of one or more precious substances contained in blood such as represented by, for examaple, gaseous components such as oxygen and carbon dioxide, ions such as hydrogen, sodium, potassium, calcium and chlorine, or compositions such as glucose, urea, uric acid and creatinine, is nowadays one of the important therapeutic procedures necessary to monitor the pathological status of a patient, to administrate the anesthesia during a surgical operation and/or to monitor the therapeutic effect. The method for the continuous measurement of the chemcial properties of blood components broadly includes that for in vivo application wherein a chemical component sensor device is implanted or introduced percutaneously and that for ex vivo application wherein the chemical component sensor device is disposed in an extracorporeal circuit and blood is introduced to the sensor device.
The in vivo system is effective to measure the concentration of a chemcial component of interest at a particular site of the blood vessel. However, since the chemical component sensor device is required to be indwelled or implanted in the blood vessel, and, on the other hand, since the size of the available chemical component sensor device is limited, it is difficult to manufacture the chemical component sensor device that is selectively sensitive to a plurality of substances contained in the blood. Moreover, while it is well known that the indwelling component sensor device is constantly in contact with the blood within the blood vessel and is susceptible to a drift in output characteristics as a result of adsorption of protein, correction cannot be effected without difficulty once the component sensor device has been indwelled in the blood vessel. In view of these problems, the in vivo system has not yet found wide applications in a clinical situations.
In contrast thereto, the ex vivo system is expected to find an increasing application. In particular, since the component sensor device is used ex vivo and, therefore, the size of the component sensor device does not matter, the component sensor device which is selectively sensitive to a plurality of substances can easily be made. Further, since an automatic drift correction can easily be embodied and the component sensor device can be regularly cleansed, a stabilized measurement for a prolonged time is possible.
The ex vivo blood component monitoring apparatus, that is, the blood component monitoring apparatus using the sensor device for ex vivo application, is currently available in two types; a blood drain type and a blood recirculating type. In the blood drain type blood extracted from the blood vessel for the measurement is drained out of the system after the measurement has been done and has merit in that neither the component sensor device nor a correcting fluid need by sterilized, but is deficient in that the valuable blood used for the measurement is discarded. On the other hand, in the blood recirculating type, the blood extracted from the blood vessel for the measurement is circulated back to the blood vessel after the measurement has been made and has merit in that the valuable blood is returned to the patient without being drained out of the system, although there are requirements that the component sensor device must be sterilized and the correcting fluid should, because it is infused into the blood vessel together with the recirculated blood, be sterile and harmless to the patient.
The ex vivo blood component monitoring apparatus of blood drain type includes, for example, a pH, PCO.sub.2, and PO.sub.2 monitor disclosed by J. S. Clark et al. in 1971 (See Computers and Biomedical Research 4, 262 [1971]) and a monitor for the analysis of blood potassium, sodium, calcium and hydrogen ions disclosed by A. Sibbald et al. in 1985 (See Medical and Biological Engineering & Computing 23, 329 [1985]). In these prior art monitoring apparatuses, a multi-chemical sensitive sensor device used therein is disposed in a constant temperature bath fluid-connected with the indwelling catheter through a tubing for the introduction of blood from the indwelling catheter to the constant temperature bath during the measurement, the blood being discarded out of the system after the actual measurement. A major disadvantage inherent in the ex vivo monitoring apparatus of the blood drain type lies in that the blood is discarded. Except for the cases receiving a surgical operation while being blood-transfused, it can hardly be regarded tolerable to drain a few tens of milliliters of blood per day out of the system from a severely ill patient only for the purpose of measurement of chemical properties of substances contained in the blood.
An example of the ex vivo monitoring apparatus of blood recirculating type includes a blood component monitoring apparatus of the transfusion type disclosed in, for example, the Japanese Laid-open Patent Publication No. 55-76639 published June 9, 1980, the invention of which has been assigned to the same assignee of the present invention. According to this publication, the chemical sensitive sensor device is mounted in an indwelling catheter indwelled in the blood vessel and fluid-connected with a transfusion tubing through which a physiologically compatible liquid being transfused for the correction of the sensor device is supplied, and blood is extracted into the sensor device at any desired time during the transfusion taking place for the measurement of the chemical properties of substances contained in the blood. The blood so extracted into the sensor device is, after the measurement, circulated back to the blood vessel together with the physiologically compatible liquid being transfused. The monitoring apparatus of transfusion type referred to above has subsequently been improved, the improved versions of which are disclosed in the Japanese Laid-open Patent Publications No. 59-155240 and No. 60-116332 published Sept. 4, 1984, and June 22, 1985, respectively.
During the course of a series of experiments conducted on animals with the use of the above described monitoring apparatus of the transfusion type with a view to commercializing the apparatus, the inventors of the present invention have encountered with the following problems.
(1) Even in the system, such as in the above described monitoring apparatus of the transfusion type, wherein the measurement is carried out while blood is intermittently extracted, adsorption of protein, blood cells, fibrinogen and some other substances contained in the blood to the chemical sensitive sensor device occurs with the consequence that the response of the chemical sensitive sensor device tends to be lowered, even though the degree of adsorption is small as compared with that exhibited by the chemical sensitive sensor device for in vivo application wherein constant contact between the chemcial sensitive sensor device and the blood takes place.
(2) Since the temperature of the site at which detection is made tends to change during a single pumping cycle during which the blood is extracted and then circulated back to the blood vessel, the temperature dependent change of the chemical sensitive sensor device must be compensated for.
(3) While the particular parameters of the chemical sensitive sensor device are determined with the use of a control solution prepared to provide a replica of blood plasma, it has been found that, even though the concentration of chemical components remains substantially the same between the control solution and the actual blood, the chemical sensitive sensor device tends to give different output characteristics between the control solution and the actual blood during the same pumping cycle, and therefore, this leads to an error occurring during the actual measurement of the chemical properties of the substances contained in the blood.
The problem (1) discussed above may be obviated substantially if the chemical sensitive sensor device used is frequently replaced with fresh sensor devices of identical construction to avoid the adverse influence brought about by the absorption of the chemical substances, and the problem (3) discussed above may not be crucial in the case of the chemical sensitive sensor device for use in the measurement of PCO.sub.2 which exhibits a relatively small change in output characteristic between the control solution and the actual blood.